Immunosuppressive Activity of an Entomopathogenic Bacteria, Xenorhabdus ehlersii KSY, and Its Application to Enhance Insecticidal Activity of Bacillus thuringiensis

摘要

An entomopathogenic bacterium, Xenorhabdus ehlersii KSY, is symbiotic to a nematode, Steinemema longicaudum, and exhibits high entomopathogenic virulence against lepidopteran insects. This study showed that the bacterial pathogenicity is induced by its inhibitory activity against eicosanoid biosynthesis of target insects, resulting in immunosuppression. To be applied for insect pest control, the bacteria should be infected to insect hemocoel. To deliver X. ehlersii to inset hemocoel, Bacillus thuringiensis (Bt) was mixed with the bacteria to breakdown the physical barrier (= midgut epithelium) from midgut lumen to hemocoel. The bacterial mixture significantiy enhanced insecticidal activity of Bt only against larvae of Plutella xylostella and Marucavitrata. For formulation, X. ehlersii cells were freeze-dried and mixed with sporulated Bt cells. The formulated bacterial mixture was applied to semi-field cultivating cabbage crop infested by P. xylostella. The bacterial mixture treatment showed over 95% control efficacy, while Bt alone gave 80% control efficacy. These results suggest that X. ehlersii can be applied to develop a novel insect control agent.